As an example of a work vehicle having an autosteering function to be autosteered along a set target traveling path, Patent Literature 1 discloses a rice planter configured to produce a target path parallel with a teaching path produced by a teaching path producing means based on position information determined by a GPS (Global Positioning System) unit and to be caused to travel autonomously on the target path.
With this rice planter, an actual traveling path in the course of an auto traveling is determined by the GPS unit and a deviation amount between the target path and the actual traveling target path is computed for comparison with a preset threshold value. In this, an angle formed between the target path direction and the advancing direction is used as such deviation amount. And, if this deviation amount exceeds the threshold value, then, the direction of this deviation relative to a reference line is determined. If the deviation direction is deviation to the left side, a steering device will be corrected to the right direction in correspondence with the amount of this deviation. If the deviation direction is deviation to the right side, the steering device will be corrected to the left direction in correspondence with the amount of this deviation.
However, with a work vehicle implementing such principle of auto traveling function, only an angle formed between the target path direction and the advancing direction is used as traveling deviation for steering control, appropriate traveling becomes difficult in case only the amount of deviation is large, although the target path; and in case the vehicle body are located in vicinity or the amount of deviation is small, although the target path and the vehicle body are located far apart from each other.
According to a traveling control technique for a vehicle system disclosed in Patent Literature 2, for a target traveling path (a target track) including a variety of curvatures in a mixed state, a lateral deviation amount from a target traveling path, a deviation amount of direction therefrom and a curvature of the target traveling path are determined and a steering angle corresponding to the two deviation amounts and the curvature will be calculated and outputted.
Specifically, a reference point is provided such that a vehicle position may be located in a normal line direction of the target traveling path. For this coordinate system, the coordinate system is converted from an absolute coordinate system of the vehicle position into a relative coordinate system relative to the reference point. Also, deviation amounts of the relative lateral displacement and the direction angle from the target traveling path are calculated; and a steering angle is obtained under the proportional control technique, based on a feedback-like operational amount corresponding to the deviation amounts and a feedforward-like operational amount corresponding to the curvature of the target lane and the lateral displacement amount.
However, although this traveling control technique may be effective for e.g. an automobile which travels on a paved road, in the case of an agricultural work machine such as a rice planter, a tractor, a combine or a lawn mower, etc. or a civil engineering work machine such as a dozer, since the traveling speed of such machine or vehicle is low, for a same steering angle, it takes a long time for traveling deviation to be resolved as compared with a vehicle which travels at a high speed. Moreover, a momentary positional displacement from a target position may occur due to slippage or riding on an earth lump. Therefore, the traveling control technique disclosed in Patent Literature 2 cannot be applied as it is to an autosteered traveling of a work vehicle.